Active implantable medical devices typically include a generator having a metallic case that contains the various electronic circuits and an energy source (battery). At the time of the implantation, the generator is connected electrically and mechanically to an external probe that is equipped with electrodes for intra-cardiac stimulation, making it possible to detect the potentials of depolarization of the myocardium, and to deliver to the myocardium the stimulation pulses produced by the generator.
Generally, it is possible to connect a given generator to one of two different types of probes, monopolar and bipolar. This is done at the choice of the surgeon and according to the type of pathology to be treated. In the case of a monopolar (also called “unipolar”) probe, the detection and stimulation are operated between the single electrode and the metallic case of the generator. In the case of a bipolar probe, the detection and stimulation can be carried out either in a differential mode between two electrodes of the probe, or in a common mode between the case of the generator and one or the other of the electrodes. Of course, a general set of internal operating parameters of the generator must be selected according to whether the probe used is monopolar or bipolar: the commutation (switching) of the terminals internal to the generator that are to be used, the collection (detection or sensing) of the depolarization signal, the adjustment of the stimulation parameters, the modification of the control algorithms operating the microprocessor, etc.
It will be understood that any error in the selection of the type of operation (monopolar or bipolar) can involve extremely serious consequences. For example, if the device is programmed for a bipolar stimulation but is equipped with a monopolar probe, this error will cause a loss of sensing and the application of an inappropriate stimulation, with a significant risk for the patient. In addition, after the manufacture of the generator, there can be a significant lapse of time between the shipping of the generator and its implantation in the body of the patient. This time can even reach one year to eighteen months. During this sometimes lengthy storage period preceding the implantation, it is essential that the apparatus, whose battery was connected right before shipping, presents an energy consumption that is as reduced as possible, so that the storage period does not have a notable incidence on the useful lifespan of the apparatus, i.e., the duration during which it will be functional after implantation.
With this objective, the device, and in particular its microprocessor, are placed in a sleeping mode having a very low energy consumption. Moreover, it is envisaged to have a mechanism interior to the device to detect the connection of a probe. In this way, the device can detect an implantation and awaken the circuit of the generator, to make it fully functional and to initialize a certain number of parameters, to memorize the starting data, etc. This function of activation can be fulfilled in various ways. For one example U.S. Pat. No. 5,350,401 envisages an activation pin having to be withdrawn by the surgeon from the pacemaker at the time of the implantation, to authorize a communication connection with an external programmer that will send a deactivation control signal to the pacemaker to deactivate the inhibition and allow normal operation.
U.S. Pat. Nos. 5,522,856 and 5,370,666 detect the insertion of a probe by an impedance measurement that is carried out between the terminals of the connector head of the device. In the absence of a probe, the impedance is extremely high. As a probe is inserted, the impedance value decreases below a certain threshold. Crossing that threshold is detected to switch the pacemaker from the sleeping state mode to a mode where it becomes completely awake and functional.